What is a Home Drive ?

A standard configuration I see is organisations redirecting users “My Documents” to their “Home Drive”. When I ask them why I’m usually told that they don’t know why, because they’ve always done it or because they believe users “need” to see a drive letter in My Computer. Of course, you don’t HAVE to have a home drive to redirect the My Documents folder as the My Documents folder can be redirected to an UNC path (typically \servershare%username%) but you may still want to redirect them to a home drive as explained below. So, what is a home drive and why would you want to use one ?

Well, a home drive, or more correctly a home directory, is a special type of mapped drive that contains a users folders and can contain application data. It allows programmatic access to the home drive by assigning values to the variables:

  • HOMEDRIVE
  • HOMEPATH
  • HOMESHARE

For example, these three environment variables could contain the following:

HOMEDRIVE=<drive letter>:
HOMEPATH=<path>
HOMESHARE=\<server name><share name>

The home drive can then be accessed in a standard logon script. Below are some parameters that can be used and their meanings.

 

Parameter
Description
%HOMEDRIVE% The user’s local workstation drive letter connected to the user’s home directory
%HOMEPATH% The full path of the user’s home directory
%HOMESHARE% The share name containing the user’s home directory

 

So, we can assign a home drive rather than a standard “mapped” drive to enable us to reference the drive in scripts. But, is that all that using a home drive gives us ? The answer is “no”. If you have not assigned a home drive to a user in their Active Directory object then Windows (on clients) uses a default location, the users profile in Documents and Settings / Users directory for files and for user-specific application files such as .ini files it uses the users Windows directory which, be default, is the Windows directory on the client. Therefore one thing extra that using a home drive gives us (over a standard mapped drive) is a place to store user-specific application settings which will follow the user from machine to machine – note that this is distinct from roaming user profiles as these files are not stored in the users profile by default. Also, as the default home location is the users My Documents folder in their profile if we map a home directory we change the home location. This doesn’t mean that the users My Documents location is changed but it does mean that the default location for Open, Save As and command prompt start points is the users Home Directory.

From the above you can see that if we set a users home directory (Home Drive) to be H: then when they try to save a file in Microsoft Word, for example, it will offer to save the file to H: by default. It is for this reason that you often see My Documents redirected to the home drive location….. so that users will save to their My Documents location by default.

As you can see from the above a Home Drive is not just another mapped drive but has a real affect on the end user experience and where files are saved.

Should I virtualise my Domain Controllers ?

Now that’s a difficult question. If you asked me “Can I virtualise my Domain Controllers” then that’s a different question to which the answer is “Of course, its fully supported depending on your virtualisation platform and the version of Windows being used but if you’re on the latest Hyper-V and the latest Windows then its fine”. The question “Should I virtualise my Domain Controllers ?” recognises that you can but that you have a choice as to whether you do or not and, as with any IT decision, you should research, size and plan. What I’d like to talk about today is two items to consider when thinking of virtualising domain controllers.

The first is around synchronisation of system clocks. As mentioned in a previous article windows Servers use time synchronisation to ensure against replay attacks and thus increase the security of Kerberos authentication within an Active Directory environment. However, virtual platforms such as VMWare or Hyper-V also allow you to synchronise a virtual machines clock with the physical host. What this means though is that, if the server host is showing a different time from the root PDC Emulator then any virtualised domain member server or domain controller will set its clock against the domain and then set its clock against the physical host and then against the domain and then against the physical host and so on ad nauseum. This can cause five issues:

  1. If there is more than the amount of “difference” between the DC clock and other domain controller clocks then the server will not be able to synchronise
  2. Similarly, as the DC clock will different from those of clients, clients will fail authentication against this domain controller.
  3. This constant re-synchronisation will cause clock “flapping” so that any events or logs written will have events recorded in an incorrect order. This is an issue not only for domain controllers but also for other servers such as SQL or Exchange where they record the time of records being changed or messages arriving.
  4. If you run an environment where accurate times are important then this will into be possible with “flapping” clocks. For example, if you require staff to “clock in” and penalise them for late arrival then your solution will be at risk if your clock cannot keep accurate time.

So, by all means virtualise your domain controllers but don’t allow them to synchronise their clocks with the physical host. In Hyper-V this behaviour can be disabled by opening the Hyper-V Manager Console. selecting the virtual machine and clicking on Settings in the Actions pane for that virtual machine. Under the Management node select Integration Services and clear the Time Synchronization check box.

 

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Click on Apply and that virtual machine will now synchronise its clock solely based on the settings within its operating system.

The second item to consider before virtualising your domain controllers concerns “snapshotting”. Snapshots allow you to take a point in time view of a server and then record differences to the virtual disk of that server over time. In this way you can “roll back” a virtual machine to the point the snap shot was taken by removing the changes made. However, this gives an issue when we consider domain controllers.

When a change is made on a Domain Controller it updates its own Update Sequence Number (USN) and, when a synchronisation is due with other domain controllers, issue the update to them. These USN’s are maintained per Domain Controller and a certain change may register on DC1 as 12345 and hold the USN of 7657622 on the far older DC2. You can see the USN on a particular Domain Controller by looking at the highestCommittedUSN value using ADSIEdit to connect to the RootDSE default naming context.

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DC1 would look like above and DC2 would have the USN below, for example.
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Now, it’s a basic premise that the USN on a domain controller should only ever get bigger, and not smaller. After all, transactions can’t just disappear. Indeed, domain controllers use this USN to keep track of the updates they have received from each other. The last USN received from each replicating partner is stored in a High Watermark Vector Table on each DC. In this way, the receiving domain controller knows which was the last change it received form a replicating partner. When it next wants to replicate it sends its high watermark value to the DC it wants to replicate from (the source domain controller). The source DC then uses the information in the high watermark value to determine which objects to replicate back to the target Domain Controller. This can be represented by the following table:

Step DC USN High Watermark Value Action
1 DC1 100 200 Initial Value
DC2 200 100
2 DC1 108 200 Changes made on DC1 (New user created for example)
DC2 200 100
3 DC1 108 200 DC2 requests changes, synchronises and updates it high watermark value for DC1
DC2 200 108
4 DC1 127 200 Further changes are made on DC1
DC2 200 108
5 DC1 127 200 Only changes 109 to 127 are synchronised to DC2
DC2 200 127

 

So far so good. So, what’s the issue. The issue is that if we had taken a snapshot of DC1 at, say, step 3 and rolled back then the following would happen.

Step DC USN High Watermark Value Action
1 DC1 100 200 Initial Value
DC2 200 100
2 DC1 108 200 Changes made on DC1 (New user created for example)
DC2 200 100
3 DC1 108 200 DC2 requests changes, synchronises and updates it high watermark value for DC1
DC2 200 108
4 DC1 127 200 Further changes are made on DC1
DC2 200 108
5 DC1 127 200 Only changes 109 to 127 are synchronised to DC2
DC2 200 127
6 DC1 108 200 Active Directory database “restored” on DC1
DC2 200 127
7 DC1 119 200 Further updates made on DC1 raising its USN past the old value of 127
DC2 200 127
8 DC1 147 200 DC2 requests changes past 127 – DC1 send changes 128 to 147 – the “new” changes in the range 109 to 127 are lost and never synchronised
DC2 200 127

 

So, by restoring Active Directory from a snapshot we would run the risk of losing updates IF Active Directory allowed us to do this. Fortunately the clever guys at Microsoft have worked this out and from Windows 2003 SP1 this is not likely to happen because AD will recognise that the USN’s have become out of sequence and will refuse to allow DC1 to synchronise. You will know if this has happened to you not only because your domain will not synchronise properly but you will see an event similar to the below logged in the event viewer on the “restored” Domain Controller.

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 As you can see, the only solution for this is to forcibly demote the domain controller and start again. Of course, the situation is even worse if ALL domain controllers are snapshotted and then restored. It’s perfectly possible that you can end up without an operating Active Directory environment ! So, the original question was “Should I virtualise my Domain Controllers ?” and I say that this is a decision that you have to make yourself and the risk you want to assume. However, I would suggest that a best practice is to:

  • Never synchronise Domain Controller clocks with the virtualisation host
  • Never snapshot domain controllers
  • Always have at least one (and preferably two) physical domain controllers in case you have to force demote all virtualised domain controllers

If you follow the above advice I believe the risks in virutalising DC’s are relatively low.

Synchronising time in an Active Directory Forest

Windows Servers use time synchronisation to ensure against replay attacks and thus increase the security of Kerberos authentication within an Active Directory environment. Kerberos tickets are presented to domain controllers by clients and the authenticating domain controller checks that the time stamp on the ticket is within a certain amount of time of the clock on the domain controller, generally 5 minutes although this can be set by group policy. To ensure that the clocks between clients and domain controllers are reasonably in synch at all times, Windows operating systems use the windows time service (W32Time) to synchronise clocks within the forest. They do this by following a synchronisation hierarchy which can be described as below.

  • All client desktop computers nominate the authenticating domain controller as their in-bound time partner.
  • All member servers also nominate the authenticating domain controller as their in-bound time partner.
  • All domain controllers in a domain nominate the primary domain controller (PDC) operations master as their in-bound time partner.
  • All PDC operations masters follow the hierarchy of domains in the selection of their in-bound time partner and synchronise with the server holding the PDC operations master role in the forest root domain.
  • In this hierarchy, the PDC operations master at the root of the forest becomes authoritative for the organization and should be set to synchronise with an external atomic time source or will use its own CMOS clock to set its internal time.

When deploying domain controllers, the server holding the PDC operations master role must then be granted access to the internet to synchronise its time with an atomic clock using the NTP protocol (UDP port 123). This is fine when a data centre is first deployed but what happens when the PDC emulator operations master role is moved to another server ? Synchronisation will still occur within the forest with the new server holding this role other than for the original server hosting the role which will still synchronise with its external source.

This can raise two issues.

  1. One of the servers, over time, could drift in terms of its clock and so the original PDC emulator may not be in sync with the rest of the forest and therefore reject authentication attempts.
  2. If an application is Active Directory integrated and using the AD servers for its time source then that application or service may not be accurately recording its time in the application or any logs. This can be an issue, for example, for systems “clocking in” staff where staff are fined for late arrival or, perhaps, when accurate recording of access times for network assets is important for security reasons. If the time recorded is out by a few minutes then this may be an issue.

This raises the question then “how do I set my clocks accurately for all machines if I move my PDC Emulator role”. There are three solutions.

  1. Set all domain controllers to synchronise to the same external atomic clock. Whilst this should certainly keep all clocks within a reasonable skew time the same issue can occur as described above if one server should, for some reason, not have external access. It is likely to eventually drift from the other server clocks and create issues.
  2. Set two domain controllers to synchronise externally and only move the PDC emulator role between those two servers. This is a standard answer to this conundrum and can work well. It has the advantage that administrators are likely to know where the FSMO roles are for the domain and reduces the amount of setup time. However, there is still a small risk of drift if one server loses its external access for any reason. There is also the possibility that the PDC Emulator role will be moved to a server other than those nominated.
  3. Use a GPO to set domain controllers to follow the hierarchy above and be compliant with RFC 1305 unless they are the PDC Emulator in which case they go to an external source. While all domain controllers still need to be provided access to the internet over UDP port 123, should a mistake be made then all servers will at least have the same clock time and so, using this method, the likelihood of failed authentication attempts are very much minimised.

The preferred method then is option C above. To configure this is relatively simplistic. A GPO is created and applied at the “Domain Controllers” OU level. The GPO itself is scoped by way of a WMI script to affect just the server holding the PDC Emulator operations master role. This GPO configures the W32Time service on that server with the external clocks to synchronise to.

To create the GPO, open Group Policy Management Console and create a new GOP linked to the Domain Controllers OU (I have called mine PDC Time Sync). Access the Scope tab of the GPO.

Note that WMI Filtering is net to <none>. You can only apply a WMI filter if one exists and so, next, we right click the WMI Filters node above and select “New…“. We give the WMI filter a name and description as below.

We then click Add to add a query to the filter. The WMI query will be as below:

Select * from Win32_ComputerSystem where DomainRole = 5

We then click on OK and “Save“.

The WMI script selects computers whose DomainRole method of the Win32_ComputerSystem class (i.e. DomainRole value) is set to 5. The allowed values for this method are as below.

Value Meaning
0 Standalone Workstation
1 Member Workstation
2 Standalone Server
3 Member Server
4 Backup Domain Controller
5 Primary Domain Controller

 

Active Directory follows the multi-master method of replication whereby each domain controller “owns” a copy of the Active Directory database and can update values in that database and replicate changes to all other domain controllers. This is as opposed to the NT4 methodology where a primary domain controller existed and all other domain controllers were backup domain controllers. However, Active Directory maintains the PDC Emulator role for those times when a PDC is still required for down level clients, password replication and time synchronisation. The server hosting this role has DomainRole 5 and all other domain controllers hold domain role 4, even though they are not backup domain controllers in the traditional meaning of that term.

As the WMI filter now exists it can be assigned to the GPO as a filter. Access the scope tab of the GPO created earlier and set the WMI filter to the one just created.

The GPO can now be edited and values set to control the W32Time service on the server holding the PDC Emulator role. If the role is moved between servers then the GPO ceases to apply and the W32Time settings are reverted to their original values forcing the server to sync to the new server holding the PDC Emulator role. The appropriate policy to set is the “Configure Windows NTP Client” policy found at “Computer Configuration | Policies | Administrative Templates | System | Windows Time Service | Time Providers” in Windows Server 2008 R2. The default values for these settings are shown below.

The meaning of each value is as follows:

Policy Setting Effect of Setting
NtpServer
Establishes a space-delimited list of peers from which a computer obtains time stamps, consisting of one or more DNS names or IP addresses per line. Computers connected to a domain must synchronize with a more reliable time source, such as the official U.S. time clock. This setting is used only when Type is set to NTP or AllSync.

0x01 SpecialInterval

0x02 UseAsFallbackOnly

0x04 SymmetricActive

0x08 NTP request in Client mode

Type
Indicates which peers to accept synchronization from:

NoSync. The time service does not synchronize with other sources.

NTP. The time service synchronizes from the servers specified in the NtpServer registry entry.

NT5DS. The time service synchronizes from the domain hierarchy.

AllSync. The time service uses all the available synchronization mechanisms.

CrossSiteSyncFlags
Determines whether the service chooses synchronization partners outside the domain of the computer.

None               0

PdcOnly          1

All                   2

This value is ignored if the NT5DS value is not set.

ResolvePeerBackoffMinutes
Specifies the initial interval to wait, in minutes, before attempting to locate a peer to synchronize with. If the Windows Time Service cannot successfully synchronize with a time source, it will keep retrying, using the settings specified in ResolvePeerBackOffMinutes and ResolvePeerBackoffMaxTimes.
ResolvePeerBackoffMaxTimes
Specifies the maximum number of times to double the wait interval when repeated attempts fail to locate a peer to synchronize with. A value of zero means that the wait interval is always the initial interval in ResolvePeerBackoffMinutes.
SpecialPollInterval
Specifies the special poll interval in seconds for peers that have been configured manually. When a special poll is enabled, Windows Time Service will use this poll interval instead of a dynamic one that is determined by synchronization algorithms built into Windows Time Service.

 

A list of NTP Time servers can be obtained from http://support.microsoft.com/?id=262680. The values I tend to set for the GPO are as below.

I use the flag 0x1 on the NtpServer setting to make the time service take note of the SpecialPollInterval setting (which is a value in seconds) which sets how often the server should poll for a new time. In this way we can, if desired, poll for a time update more or less often. The NTP type setting tells the service to go direct for its time updates to the NtpServer specified.

One the GPO is configured you can wait for it to be applied to the PDC Emulator or force its application using GPUpdate. To check that the policy is working simply note the difference between the server clock and your wristwatch or other clock and, around an hour later, check again and you should see that the difference between the two time sources has changed as the server is drawing its time from an accurate time source.

The final thing to do is to change the GPO status to disable user configuration settings within the GPO as this will lead to a slightly faster GPO processing time.

 

I hope this post will help remove some of the mystery surrounding how to configure time synchronisation settings within an Active Directory domain.